Any discussion of the prior art throughout the specification is not an admission that such prior art is widely known or forms part of the common general knowledge in the field.
It has been found that a granular stone column can be formed reliably by using a drill unit with concentric inner and outer drills, the outer drill including a cavity in which the inner drill lies. Drills of this type are described in JP 62-228514, JP 59-141622, JP02-167918, WO2010/029871 or the applicant's own invention described in PCT/IB2012/051585. The inner drill feeds the granular material used to form the column out of an open end of the outer drill as the stone column drill is extracted from the ground. The feed rate of the stone column drill and inner drill can be adjusted to vary the properties of the column formed, but, there are limits to this approach and some granular material can move, or be forced, inwardly depending on the feed rates, rotational directions and speeds of the inner and outer drills. If some granular material does move inwardly, or against the direction of flow of the bulk material, then this can detrimentally affect the quality of the granular stone column formed and/or increase the likelihood of bridging occurring and/or reduce the feed rate of granular material into the column through the drill unit.
It is possible to reduce or eliminate the chances of bridging and modify the characteristics of the granular stone column formed by using the applicant's invention described in PCT/IB2013/091395 but this can add to the cost of the stone column drill and may still not prevent some granular material moving inwardly, slowly or against the bulk direction of flow of the granular material.
When installing a group of columns in close proximity in ground susceptible to liquefaction, or consisting of a fluid soil, as more columns are installed the ground appears to pressurise and this can prevent the column material from being fed properly or successfully compacted.